Decontamination and treatment of radwastes, accumulated in the course of industrial production of military plutonium in the United States as well as in Russia, is an important subject of investigation and research. Processing and disposal of Hanford Site high-level waste (HLW), containing about 329,000 tons of alkaline solutions, salt cakes and sludges with considerable quantities of hazardous long-lived plutonium, neptunium and americium isotopes, is part of the problem. The chemical composition of the Hanford Site wastes is complex and varies greatly from one tank to another.
It is recognized that most of the transuranium radionuclides likely will report to the solid phase following processing of Hanford HLW. However, even small quantities of these radionuclides remaining in the product supernate solutions may be enough that the alkaline solutions cannot be classified as low-level wastes (LLW). Additional purification of alkaline solutions from transuranium elements (TUE) likely will be required. Special methods, based on the coprecipitation of TUE with convenient carriers, is a probable solution to this problem. It is expected that the precipitates obtained by coprecipitation will be suitable for vitrification or for other subsequent treatments to isolate the TUE.
Coprecipitation of TUE with various carriers has been the subject of numerous investigations and is widely used in analytical practice, as part of certain radiochemical technologies, and as a treatment method in ecological settings. Metal hydroxides are potential carriers to purify alkaline solutions of TUE. Certain types of compounds having low solubility at high hydroxide (OH.sup.-) concentrations can be used for the coprecipitation. However, application of these carriers in the ordinary way (i.e., by the introduction of low acid or neutral aqueous solutions of the compound directly into the alkaline medium) is complicated by the rapid formation of the precipitate at the boundary surface between the mixed liquids. As a result, TUE isolation occurs mainly by adsorption on the surface of freshly formed precipitates and the effectiveness of TUE capture is low.
Compounds which are soluble in alkaline solution, but which can be readily precipitated by hydrolysis or redox reactions, avoid these difficulties. Such an approach (named the Method of Appearing Reagents--MAR) should accomplish uniform formation of the precipitate throughout the entire solution volume and provide more effective capture of TUE by the precipitating solid phase.